1. Field of the Invention
The present disclosure relates generally to the detecting of explosive substances, and more particularly to the detecting of explosive substances within remote targets and the locating of such targets.
2. Related Art
An improvised explosive device (IED) is an explosive device placed or fabricated in an improvised manner, often used in unconventional warfare by terrorists or guerrillas. These IEDs are sometimes referred to as roadside or car bombs. The ever-increasing need to protect soldiers and civilians alike has resulted in demand for explosives detection systems that can detect and locate an IED at a significant standoff distance—ideally, near or beyond the IED's kill radius.
It is well known that explosives can be detected by bombarding them with thermal or slow neutrons of kinetic energy levels of approximately 0.026 eV, then detecting the resulting gamma rays. The vast majority of conventional chemical explosives are nitrogen-14 (14N) rich, while Special Nuclear Materials (SNMs) may contain Plutonium-239 (239Pu), Uranium-235 (235U), or, both as key ingredients. Each of these constituent elements, 14N, 239Pu, and 235U, as well as other materials of interest in detecting improvised explosives, for example, chlorine, copper, and aluminum, among others, radiates its own characteristic gamma ray emission spectrum when dosed with thermal neutrons.
For instance, militarily significant conventional chemical explosives, which constitute by far the largest threat to human life in terms of the frequency of occurrence, historical lethality, and ease of procurement and use, contain very high densities of nitrogen, principally nitrogen-14. Nitrogen-14, when bombarded by a thermal neutron, emits a strong gamma ray with energy 10.83 MeV as follows:14N+1n→15N+γ  [gamma ray]
The gamma ray emission is isotropic in that it can be emitted in any direction, and its trajectory is uncorrelated to the trajectory of the incident thermal neutron. High gamma ray fluxes are interpreted as explosives detection events. This technique is known as Thermal Neutron Activation Analysis (TNAA).
TNAA is a well-known technique for explosives detection and other types of materials analysis. However, the majority of TNAA technology has been directed at explosives detection in luggage and at landmine detection. Both applications operate in environments with complicating factors that limit the success of TNAA. Many common items found in luggage, such as nylon sweaters, are rich in nitrogen. This reduces the signal-to-noise ratio (SNR), which increases the false alarm rate and lowers the overall detection rate. Explosives distributed in small pieces in luggage also reduce the SNR and the detection rate in TNAA. Likewise, the most significant issue with landmine and buried explosives detection is the presence of significant amounts of silicon-29, which constitutes up to 5% of native silicon, and which emits gamma rays at 10.6 MeV under thermal neutron dosing. This emission (noise) competes with the gamma rays from nitrogen at 10.83 MeV (signal), reducing the SNR, increasing the false alarm rate, and decreasing the overall detection rate. Furthermore, the reduced SNR in both applications translates into increased inspection times and decreased throughout.
By contrast, in accordance with embodiments of the present invention, the proposed use of TNAA for IED detection operates in more conducive environments. First, the most deadly IEDs contain significant amounts of explosives, and hence, of nitrogen, since they are very large compared to anti-personnel land mines or to bombs in luggage. Thus, the targeted signal is high compared to competing signals from other noise sources from environmental nitrogen-14 and silicon-29. This has the effect of improving the SNR, decreasing the false alarm rate, and increasing the overall detection rate. Second, IEDs are often placed at or above the ground or buried with shallow overburden when compared to their explosive weight; they are often buried in trash piles or placed near concrete or dirt roads. Although silicon is present in these environments, its effect is significantly smaller than that in the case of buried antipersonnel mines with amounts of explosives measured in ounces. A typical IED is a command detonated device whose primary component is one or more HE (high explosive) 155 mm (U.S./NATO) or 152 mm (Soviet/WTO/Indigenous copy) (diameter) artillery rounds consisting of a metal casing filled with high explosive and measuring about 450 mm long. Increasingly, so-called “home made explosives” [HMEs] are also found, often consisting of ammonium nitrate plus fuel oil, urea nitrate, and similar nitrogen-rich compounds.
More importantly, there is presently no device for effectively detecting and locating IEDs at a standoff distance. Landmine detection and explosives detection in luggage both examine targets at close proximity. Therefore, a further object of the present invention is to detect IEDs using TNAA under different conditions, and thereby significantly reduce friendly and civilian casualties. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.